The present invention relates to an improved gravity gradiometer instrument (GGI) and, more particularly, to gravity gradiometer instruments that are responsive to one or more higher-order gravity-gradient characteristics.
Various instruments have been developed to measure gravity gradients, these instruments include gradiometers that are designed to measure the differential curvature or ellipticity of gravity equipotential surfaces, the rate of change of the increase of gravity in the horizontal direction, and/or the rate of increase of gravity in the vertical direction.
Gradiometers have been used as navigational aids in sub-surface sea-going vessels, gravity field surveys in which one or more gradiometers are carried in a vehicle (i.e., aircraft, surface or sub-surface sea-going vessel, land vehicle, etc.) and, more specifically, as an aid in identifying the boundaries of sub-surface liquid hydrocarbon deposits.
A representative or example gradiometer is shown in FIG. 5 and is sold by the Lockheed Martin Corporation (Niagara Falls N.Y. USA) and is described in more detail in U.S. Pat. No. 5,357,802 issued Oct. 25, 1994 to Hofineyer and Affleck and entitled “Rotating Accelerometer Gradiometer,” the disclosure of which is incorporated herein by reference.
As shown in FIG. 5, the exemplary gravity gradiometer instrument GGI includes eight accelerometers 100 mounted at a common radius and equi-spaced about the periphery of a rotor assembly 102 that is rotated at a constant and controlled angular velocity about a spin axis SAx. The rotor assembly 102 includes the rotor 104 carried on a support shaft 106 for rotation therewith. The rotor assembly 102 is rotatably mounted in ball bearings 108 and, in turn, carried in a flex-mount assembly 110 and carried in a gyro-stabilized gimbal mount. Processing electronics 112 are mounted on the rotor 104 adjacent each accelerometer 100 for processing the respective accelerometer output signal. An inner housing 114 contains the rotor assembly 102 and is designed to rotate with the rotor assembly 102. An outer housing 116 contains the interior components and includes one or more heaters 118 designed to operate the instrument at some controlled temperature above ambient and also includes a magnetic-field shield 120. A slip-ring assembly 122 at the upper end of the mounting shaft 106 provides the electrical/signal interface with the rotor assembly 102 and the active devices thereon. A shaft encoder 124 at the lower end of the mounting shaft 106 cooperates with an encoder pick-off 126 to provide rotary position information. The output of the encoder pick-off 126 is provided to a soft/firmware-controlled computer or microcomputer and speed controller, which, in turn, controls a drive motor 128 at the upper end of the unit to provide a controlled rotary velocity.
The gradiometer includes an internal linear servo controlled actuator that imparts a 2 Hz sinusoidal acceleration to each accelerometer pair to enable biasing and compensation of various errors including the g2 rectification error. In addition, the gravity gradiometer GGI is mounted on an external vibration isolation system that assists in attenuating higher frequency vibration.
Each accelerometer 100 is of the force-rebalance type and provides a substantially sinusoidally varying analog output that is a function of the acceleration experienced by each accelerometer as the accelerometer orbits the spin axis SA. For a gradiometer having its spin axis SA aligned along the field lines in an ideally uniform and unperturbed gravity field, each accelerometer experiences the same acceleration forces as its proceeds along its orbital path. However, when the local gravity field is perturbed by the presence of one or more masses and/or the spin axis SA is tilted relative to the local vertical field lines, each accelerometer will experience different accelerations throughout its respective orbit about the spin axis SA.
Gradiometers have typically been positioned with their spin axis vertical (VSA—Vertical Spin Axis), their spin axis horizontal (HSA—Horizontal Spin Axis), and in a three-GGI cluster at an ‘umbrella’ angle in which the spin axis is tilted 35 degrees from the local vertical, though any orientation is possible. The quantitative output of each rotating accelerometer pair, when summed and differenced, can be used to provide information related to the local gravity gradient field.
Gradiometers measure the second-order variation of gravitational potential and currently do not directly measure or otherwise determine third, fourth, or higher-order effects. Knowledge of the second-order effects can be used, for example, in verifying the veracity of the primary gradient measurement in submarine navigation systems, especially in those cases were the submarine is navigating along an iso-gradient line (wherein the second-order data would be zero, i.e., the partial derivative of the first-order gradient in the direction of movement would be zero). Additionally, knowledge of the second-order characteristics can be useful for edge detection of buried objects or bodies and fluid boundary detection, e.g., in resource exploration. Third-order gravity tensor components provide a natural filtering or upward continuation that may be useful for profiling objects close in proximity to the measuring gradiometer device. This is advantageous because background objects now only influence output data as inverse distance to the fourth power. Likewise, fourth-order gravity components filter even more background “clutter” by signal naturally rolling off proportional to inverse distance to the fifth power.